A transmission flush is not really necessary especially if your vehicle has under 100,000 miles on it.
Most transmission shops and auto shops recommend flushing your transmission out every 10,000 to 30,000 miles or before 45,000 miles.
But you really don't have to do that as the transmission fluid is likely still good until you reach 100,000 miles.
Actually if you're gonna have a transmission flush you really should just change the transmission fluid along with the transmission filter.
The transmission fluid and transmission filter though should be changed at around 20,000 to 30,000 miles but some can wait until the car reaches 100,000 miles.
Me personally I change transmission fluid and transmission filters in my own vehicles transmissions at around 20,000 to 30,000 miles.
Doing that helps extend the life of the transmission.